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H e a r t B e a t s

DDifferentiating between right and left bundle branchblocks is important in identifying and treating yourpatient’s underlying condition and improving patientoutcomes. Bundle branch blocks are a relativelyfrequent finding in a patient’s ECG. They can beclinically insignificant and asymptomatic, or theycan signal serious underlying cardiac disease. Let’sstart by reviewing the normal electrical conductionsystem of the heart, and explaining the significanceof the different types of bundle branch blocks.

Cardiac conduction systemCardiac cells, in their resting state, are electricallypolarized. This electrical polarity is maintained bymembrane pumps that ensure the appropriatedistribution of ions (primarily potassium, sodium,chloride, and calcium). The exchange of electrolytescauses depolarization and repolarization that resultsin myocardial contraction (systole) and relaxation(diastole). Electrical signals that cause your heart tobeat begin in the sinoatrial (SA) node, located at thetop of the right atrium. The SA node is also calledthe heart’s “natural pacemaker.” When an electri-cal impulse is released from this natural pacemak-er, it causes the atria to depolarize. The conductionof the electrical impulse throughout the atria isseen on the ECG as the P wave. The signal thenpasses through the atrioventricular (AV) node. Thewave of depolarization sweeps out of the AV nodeand bundle of His, into the bundle branch system.The right and left bundle branch deliver the cur-rent to the right and left ventricles, respectively.(See The anatomy of the ventricular bundle branches.)This is the most efficient means of transmittingthe electrical current. Because the left and rightventricles depolarize nearly simultaneously, theresultant QRS complex, representing ventriculardepolarization is narrow-less than 0.12 second induration. Because the muscle mass of the left ven-tricle is so much larger than that of the right ven-tricle, left ventricular electrical forces dominate

those of the right ventricle, resulting in a normalelectrical axis between 0 and 90 degrees. Therefore,bundle branch block is diagnosed by looking at thewidth and configuration of the QRS complexes.1

Right bundle branch blockIn right bundle branch block (RBBB), conductionthrough the right bundle is obstructed. As a result,right ventricular depolarization is delayed; it doesnot begin until the left ventricle is almost fullydepolarized. (See Right bundle branch block.) Thiscauses three things to happen on the ECG:1. The delay in right ventricular depolarization

prolongs the total time for ventricular depolar-ization. As a result, the QRS complex widensbeyond 0.12 second.

2. The RSR in leads V1 and V2 with ST segmentdepression and T wave inversion.

3. Reciprocal changes such as late deep S wavesseen in leads I, AVL, V5, and V6.1

Bundle branch blocksBy Nancy Femlee, BSN, and Lisa Waters, BSN

The anatomy of the ventricular bundlebranches

bundle of His

leftposterior

fascicle

right bundle branch left bundle branch

Purkinje fibers

septal fascicle

left anterior fascicle

AV node

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H e a r t B e a t s

RBBB is a common intraventricular conductionanomaly and is found in patients with and withoutstructural heart disease. Common causes includemyocardial infarction (MI), hypertensive heart dis-ease, and pulmonary diseases. RBBB can be seenin patients who have pulmonary embolism withas many as 67% showing evidence of an RBBB.Given this association, strong consideration forpulmonary embolism should be given to patientspresenting with cardiopulmonary signs and symp-toms with evidence of a new (or not known to beold) RBBB. RBBB may occur in patients who haveacute MI and is estimated to occur in 3% to 7% ofpatients. Coronary artery disease is among manycauses of new RBBB.2

Left bundle branch blockIn left bundle branch block (LBBB), left ventriculardepolarization is delayed. ECG criteria are:1. QRS complex widened to greater than 0.12 second2. broad or notched tall R waves with prolonged

upstroke in leads I, AVL, V5, and V6, with STsegment depression and T wave inversion.

3. reciprocal changes in V1 and V2 consisting ofbroad, deep S waves. Left axis deviation may bepresent.1

The most common causes of LBBB include coro-nary artery disease, hypertension, and cardiomy-opathy. Other more rare causes include Lev disease(sclerosis of the cardiac skeleton) Lenegre disease(primary degenerative disease of the conduction

system), advancedrheumatic heartdisease, and calcificaortic stenosis. Thepresence of LBBBportends a poor long-term prognosis—one study showedthat 50% of patientswho have a LBBBdie within 10 years.

HemiblocksThe left bundlebranch is furthersubdivided intothree separate fasci-cles and include theseptal, anterior, and

posterior fascicles. Left anterior fascicular block(LAFB) is found in patients with and withoutstructural disease, and in isolation, is of no prog-nostic significance. It is, however, one of the mostcommon intraventricular conduction abnormali-ties in patients who have acute anterior MI and isfound in 4% of cases. When new LAFB occurs inthe presence of an anterior MI there is a slightlyincreased risk for progression to advanced heartblock. The most common vessel involved in thissetting is typically the left anterior descendingartery.2

A left posterior fascicular block (LPFB) is muchless common than LAFB, but because it suppliesthe majority of the left ventricle, it is of more con-cern. The LPFB has a dual blood supply and ismuch less vulnerable to ischemia than LAFB. Thefinding of LPFB is nonspecific and rare. It is foundmost commonly in patients who have coronaryartery disease but can be found in patients whohave hypertension and valvular heart disease. It isreportedly the least common conduction blockfound in patients who have acute MI.2 Hemiblocksare diagnosed by looking for left or right axis devia-tion. Left anterior hemiblock causes a left axisdeviation of -30 and +90 degrees. Left posteriorhemiblock causes right axis deviation.

Variations on a themeThe term, unifascicular block, is used to describeisolated LAFB, LPFB, or RBBB. Bifascicular block

Right bundle branch block1

site of right bundle branch block

V2

V3 V6

V5

V4V1

is used to describe complete LBBB or combina-tions of RBBB with LAFB or with LPFB. Lastbut not the least, a combination of RBBB withboth LAFB and LPFB is considered trifascicularblock.3

Differentiating RBBB from LBBBTo quickly differentiate RBBB from LBBB, uselead V1 and identify the direction of the terminalforce. If the complex is downward it is an LBBB,if it deflects upward it is an RBBB. Another wayto imagine this is to think of your car’s turn sig-nal, when you make a right turn (RBBB) you putthe turn signal up. When you make a left turn(LBBB) you put the turn signal down. Since theventricle supplied by the broken bundle branchgets the signal last, the last portion of the QRScomplex shows this signal. If it is the right ventri-cle that is depolarized last, then the ending signalshould be moving towards the V1 lead and pro-duce a positive QRS complex. If the left ventricledepolarized last then the ending signal should bemoving AWAY from the V1 lead and produce anegative QRS complex.

The significance of bundle branch blocksBundle branch blocks can mask an acute MI onthe 12-lead ECG. Although bundle branch blocksare not independent predictors of mortality, thepresence of transient or persistent bundle branchblock with acute MI is recognized as a marker ofincreased mortality.2

It is important to differentiate between right andleft bundle branch block especially since, unlikeRBBB, LBBB is often pathologic. In patients whohave suspected MI, meticulous interpretation of theECG is important in differentiating which patientsneed coronary reperfusion and aggressive treatment.Understanding the differences between these twoconduction defects may prevent life-threateningcomplications. ❖

REFERENCES

1. Thaler MS. The Only EKG Book You’ll Ever Need. Philadelphia, Pa:Lippinncott Williams & Wilkins; 2007.

2. Rogers RL. Intraventricular conduction abnormalities. Emerg MedClin N Am. 2006;24(1):41-51.

3. Galen S. Wagner. Marriott’s Practical Electrocardiography. 11th ed.Philadelphia, Pa: Lippinncott Williams & Wilkins; 2008.

Nancy Felmlee is a staff development specialist, Virtua Hospital, Marlton,N.J. Lisa Waters is a training coordinator, Virtua Hospital.

Left bundle branch block

site of left bundle branch block

V1

V2

V3 V6

V5

V4

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